Signal transmission system



haw/5 Aug. 15, 1933.

C. A. BODDIE SIGNAL TRANSMISSION SYSTEM Filed July 30, 1927 INVENTORClarence A.Boddie 2 r 7 AITTORNEY I J i 1 Patented Aug. 15, 1933 UNITEDSTATES PATENT OFFICE SIGNAL TRANSMISSION SYSTEM Clarence A. Boddie,Wilkinsburg, Pa., assignor to Westinghouse Electric & ManufacturingCompany, a Corporation of Pennsylvania Application July 30, 1927. SerialNo. 209,408

7 Claims.

In order to satisfactorily attain the aforementioned objects and certainothers incidental thereto, I have found it best to couple thetransmitting and receiving instruments to the over ead ground-i:ssociated with high- 60 ension po nes, rather than to the power- 'll'ieground wires, preferably Wired radio, or carrier-current signaltranstwoirrnumberf'are generally strung on the same mission overmetallic conductors is of particular importance in connection with theremote control of power-generating stations and it is also being widelyadopted for the communication of messages, either telegraphic ortelephonic, between the various units of a power-generation installationand between the substations tied-in to the power-distribution network.

In order that satisfactory point-to-point communication may bemaintained, it is customary to utilize, at each station, a frequency fortrans- 2 mitting' signals that differs materially from the frequency onwhich the said station customarily receives. It is also becoming usualto so arrange the various frequencies employed that multiplexsignal-transmission is possible, i. e., that a plurality of messages maybe simultaneously transmitted and received over the same conductorswithout interference.

During the course of numerous experiments I have made withcarrier-current installations arranged for multiplex transmission, Ihave found that the reception of signals on certain of the frequenciesemployed was seriously hampered by the presence of harmonics of othertransmission frequencies. It was often noted that the second, third, orhigher harmonics of the carrierfrequency generated at one transmittingstation coincided very closely with the fundamental frequency beinggenerated at another station, with the result that the first referred-tostation could 4 not satisfactorily communicate with other stations inthe network.

It is, accordingly, an object of my invention to provide, intransmission systems of the type described, instrumentalities andcircuit connections whereby undesired harmonics of transmittingfrequencies are prevented from interfering with the satisfactoryreception of signals.

Another, and closely related, object of my invention is to provide, inremote-control systems employing carrier-currents, means wherebyundesired harmonics of the frequencies utilized for the control of therelays at a certain substation are prevented from causing unpremeditatedactuation of analogous relays at a different substation.

poles that carry the high-tension lines, their purpose being to relievethe lines of excessive charges that may be induced thereon duringelectrical storms. To this end, the ground wires are connected to earththrough choke-coils and protective spark-gaps at a plurality of pointsalong their length, the spark-gaps being adjusted to break down underexcessive potentials. In order that the carrier currents shall not bediverted by the ground connections, I associate therewith additionalchoke-coils arranged to offer an extremely high impedance to thecarrier-frequencies.

In addition, I have greatly modified the coupling devices interposedbetween the transmitting apparatus and the ground lines by addingthereto a circuit, or a plurality of circuits, opaque to the harmonicsof the locally generated oscillations.

Among the novel features of my invention are those particularly setforth in the appended claims. The invention itself, however, both as toits organization and its method of operation, together with furtherobjects and advantages thereof, will best be understood from thefollowing description of a specific embodiment, taken in connection withthe accompanying drawing.

The single figure of the drawing is a schematic 9 view of acarrier-current transmitting and receiving system comprising a preferredembodiment of my invention.

In order that the drawing shall not be unduly complicated by thepresence of a plurality of figures which are merely duplicates of oneanother, I have not shown all of the transmitting and receiving unitsordinarily associated with a complete power network, but haveillustrated only two of such units. It is, of course, to be understoodthat, in multiplex transmitting-receiving systems of the previouslymentioned type, there will be a large number of transmittingandreceiving sets similar to the ones shown, each of which is designedto operate at certain predetermined frequencies.

Referring to the drawing, a plurality of powerlines 1, which may extendbetween any two points in a power-distribution network, are paralleled110 by a plurality of overhead ground wires 2. As

before stated, the ground wires are for the purpose of rel mving thehigh-tension lines of charges accumulated thereon, and for this purpose,they are provided, at intervals, with charge-dissipating devices 3. Eachof the charge-dissipating devices comprises an iron-core inductor 4connected in shunt to the ground wires 2: and provided with a mid-tap 5mound. A high-frequency choke coil 6 is interposed between each of theground wires and the inductor referred to, these choke coils being eachshunted by a safety spark-gap '7.

A transmitting station comprises a thermionic amplifying device 10having a filament 11, a plate 12 and a grid 13, the filament beingprefer ably provided with a ground connection 14. The amplifying deviceis supplied with exciting current at any desired carrier frequency froma master-oscillator 15 which is connected to the filament and gridthereof. The master-oscillator, which is shown in the drawing merely asa rectangle, may be of any desired type.

Plate potential for the amplifying device is supplied from adirect-current source 16 through an iron-core audio-frequency choke-coil1'7, and an air-core radio-frequency choke-coil 18. The output or platecircuit of the device comprises an inductor 20 shunted by a tuningcondenser 21, one end of the inductor being connected to the plate 12through a blocking condenser 22, and the mid point thereon beingconnected to the filament and ground through a conductor 23.

A thermionic modulating device 25, comprising a grid 26, a filament 27and a plate 28, is supplied with plate potential from the direct-currentsource 16 that supplies plate potential for the power-amplifying device10.

The filaments of the thermionic devices 10 and 25 are connected, inparallel to a source of filament power 30, by means of conductors 31 and32.

The secondary winding 33 of an audio-frequency transformer 34 isconnected in series in the grid circuit of the modulator tube 25, theprimary winding 35 of this transformer being energized by a microphone36 in series with a source 37 of direct-current potential.

The circuit network, including the master cscillator, the poweramplifier, the modulator and the microphone, is shown merely asillustrative of any system designed to impress modulated high-frequencycurrents upon an output circuit which may be coupled to the overheadgroundwires 2. In certain cases, the network referred to may be replacedby a much more involved system designed for multiplex transmission. Inother installations, the network may be replaced by a combinedtransmitting and receivin system equipped and arranged for break inoperation. In still other installations, the network referred to wouldbe replaced by control devices arranged to transmit a definitepredetermined series of high-frequency impulses to a distant receiver inorder to there control the operation of selector-switches, etc., which,in turn, actuate controlling-devices associated with generating,transforming or switching apparatus.

Irrespective of the manner in which the modulation or control of thegenerated high-frequency oscillations is effected, the problem ofimpressing them upon the overhead ground wires for transmission to areceiving device is substantially the same. According to my invention,the coupling arrangement interposed between the oscillation generatorand the ground wires comprises an inductor 40, the ends of which areconnected to intermediate points on the output inductor 20 through aplurality of condensers 41 and 42. The inductor 40 in turn, is connectedto the overhead ground-wires 2 through a plurality of highly insulatedcoupling-condensers 43 and 44.

The output circuit, comprising the inductor 20 and the variablecondenser 21, is tuned to the carrier frequency employed by thetransmitting station. The circuit connected thereto, which 'comprisesthe inductor 40 and the condensers 41 and 42, is also tuned to thisfrequency.

Both of these circuits are preferably so made that the decrement of eachis quite low. They are, accordingly, sharply resonant to the carrierfrequency, and quite opaque to the harmonic frequencies thereof.

In order that the transmission of harmonic frequencies to the couplingcircuit which includes the inductor 40 and the condensers 41 and 42shall be minimized, the points on the output inductor 20 to which thesaid condensers are connected may be so chosen that they aresubstantially voltage nodes with respect to the predominant harmonic.

In addition, the points on the coupling inductor 40 to which thecondensers 43 and 44 are connected may be so chosen that harmonicvoltages developed there'oetween are a minimum.

Accordingly, the harmonic frequencies are progressively attenuatedbetween the output circuit and the ground-wires 2, and are preventedfrom being impressed across the said wires with sufficient amplitude toinfluence a receivin device 50.

The number of coupling circuits interposed between the output circuit ofthe transmitting set and the transmission channel may, of course, beincreased, a single one of such circuits being shown merely asillustrative.

The illustrated receiver 50 may either be the receiver intended to benormally influenced by the transmitting device or it may be a receiverwhich would be accidentally actuated by a harmonic of the transmittingfrequency but normally actuated by the fundamentai carrier fre- Q quencyof a different transmitting station.

My invention is, as before stated, particularly adapted to multiplexcarrier-current communication systems. It is not limited thereto,however, being also useful in connection with systems wherein but a fewfrequencies are being employed for communication between definitestations, since, even in such systems, there exists the likelihood thatmore than two stations will be simultaneously communicating, and, insuch event, interference caused by harmonics might introducecomplications into both the reception of inteiiigence and the remotecontrol of machinery.

Although I have illustrated and described only 3 a single embodiment ofmy invention, many modifications thereof will be apparent to thoseskilled in the art. My invention, therefore, is not to be limited to thesimplified diagrammatic showing, and not otherwise except as limitationsmay be imposed thereon by the prior art and expressed in the appendedclaims.

I claim as my invention:

1. In combination, a guiding channel for carrier waves, a plurality ofwave generators at different fundamental frequencies coupled thereto,receiving means associated with said guiding channel and tunable toreceive at the fundamental frequency of one of said generators saidreceiver being capable of being influenced by harmonics of the generatedwave of another of said wave generators when tuned to receive at saidfundamental frequency and means for rendering said receiver free of saidinfluences, comprising devices for preventing the impressing ofharmonics of said wave upon said guiding channel.

2. In combination, a wired guiding channel for carrier waves, aplurality of means for generating carrier waves at differentfrequencies, a receiving device tunable to one of said carrier waves andcapable of being influenced by a harmonic of others of said carrierwaves, and means interposed between said generating means and thechannel for preventing said harmonic from being impressed on saidchannel.

3. In combination, a wired guiding channel for carrier waves, aplurality of means for generating carrier waves at diiferentfrequencies, a plurality of receiving devices capable of beingundesirably influenced by a corresponding generating means, at least oneof said receiving devices being capable of being undesirably influencedby a harmonic of the carrier wave generated by at least one of saidgenerating means, and means between the guiding channel and saidlast-mentioned generating means for preventing the undesired harmonicfrom affecting said receiving device.

4. In combination, a wired guiding channel for carrier waves, aplurality of means for generating carrier waves at differentfrequencies, a plurality of receiving devices capable of beinginfluenced by certain of said generating means, at least one of saidreceiving devices being capable of being undesirably influenced by aharmonic of the carrier wave generated by at least one of saidgenerating means, and means for the suppression of said harmonicinterposed between said guiding channel and the generating means givingrise to said harmonic.

5. In a signaling system, an oscillation generator having anoutput-circuit, a circuit resonant to the frequency of the oscillationsgenerated, a guiding channel, means for coupling said resonant circuitand said output circuit, and connecting means between said guidingchannel and a point on said resonant circuit that is substantially avoltage node with respect to a predominant harmonic of theoscillation-frequency.

6. In a signaling system, an oscillation generator having anoutput-circuit, a circuit resonant to the frequency of the oscillationsgenerated, a guiding channel, means for coupling said resonant circuitto said output-circuit, and connecting means between said guidingchannel and a plurality of points on said resonant circuit that aresubstantially voltage nodes with respect to a predominant harmonic ofthe oscillation-frequency.

'7. In a signaling system, an oscillation generator having anoutput-circuit, a circuit resonant to the frequency of the oscillationsgenerated, a guiding channel, means for coupling said resonant circuitand said output circuit, and connecting means between said guidingchannel and a point on said resonant circuit that is substantially avoltage node with respect to a predominant harmonic of theoscillation-frequency, said connecting means comprising devices forimpeding the passage of direct current.

CLARENCE A. BODDIE.

